When you're talking NTUs, you're talking dirty

by

Saturday, February 4th 2017

I am new to the area and have become a fan of the Mail Tribune's Fishing Report every Friday in the Oregon Outdoors section. Lately there has been some references about fishing conditions that include something called NTUs. What exactly is an NTU?

— Pete, online submission

Well, Pete, when it comes to NTUs, we're talking dirty in the Fishing Report.

Dirty, as in turbidity, or dirtiness of the water.

One of the things we've discovered over the years is that science guys have a measurement for everything, even how dirty streams such as the Rogue River can get during periods of high water.

NTU stands for — and here, Pete, is reason number 4,311 why it's better to be in newspapers than radio or television, because all we have to do is spell it, not say it — nephelometric turbidity units.

And just how do you think you get NTUs? By using, what else, a nephelometer. It's a little gizmo that shoots light through a column of water and uses sensors to count how much of the light is deflected at 90-degree angles from hitting particles.

The water nerds at Since You Asked Central recall when NTUs were actually called JTUs, or Jackson Turbidity Units. The Jackson part came from the use of a Jackson candle to measure the amount of light that passes through water, but the nephelometer took over, and the old candle got placed back in the drawer for the next power outage.

And with it came the name change. Kind of like when Mount Pitt suddenly became Mount McLoughlin. But that was well before your time here, Pete.

In low and clear conditions between storms on the upper Rogue, the river's turbidity will be around 2.5 NTUs, aka "gin clear" to anglers. In a rip-roaring storm, the caramel-colored Rogue can contain water of several hundred NTUs.

The Rogue's salmon and steelhead fishermen like the turbidity no lower than 4 NTUs and no higher than 14 NTUs.

Unfortunately, the only stream gauge where NTUs are measured on the Rogue these days is the Grants Pass water-treatment plant. You can find that data daily at www.grantspassoregon.gov/354/Water and scroll down to "River Data."

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